Teledyne Brown Engineering of
Huntsville AL, USA, is developing MUSES (Multiple User System for Earth
Sensing), an Earth imaging platform, as part of the company’s new
commercial space-based digital imaging business. MUSES hosts
earth-viewing instruments (Hosted Payloads), such as high-resolution
digital cameras, hyperspectral imagers, and provides precision pointing
and other accommodations. It hosts up to four instruments at the same
time, and offers the ability to change, upgrade, and robotically
service those instruments.

MUSES, a commercial Earth-sensing
platform on the ISS, will further increase the Space Station's research
capabilities. MUSES is is a precision-pointing platform that will mount
externally to the ISS. The instruments installed on the platform
– including high-resolution digital cameras – are oriented
towards Earth. The platform can host up to four Earth observation
instruments and offers the ability to change, upgrade, and robotically
service those instruments.

On 1 October 2013, Teledyne Brown
Engineering, Inc. (TBE), a subsidiary of Teledyne Technologies Inc.,
Thousand Oaks, CA, and DLR (German Aerospace Center — Deutsches
Zentrum für Luft- und Raumfahrt) signed a memorandum of agreement
to develop an instrument for MUSES (Multi-User System for Earth
Sensing), which will be mounted on the ISS (International Space
Station). 1)

On May 20, 2014 at the ILA Berlin
Air Show, DLR and the US corporation TBE (Teledyne Brown Engineering,
Inc.) signed an agreement to install and operate the imaging
spectrometer, DESIS (DLR Earth Sensing Imaging Spectrometer) on
board the ISS. This DLR-built instrument will be one of four camera
systems for remote sensing fitted to the MUSES (Multi-User System for
Earth Sensing) instrument carrier to be installed by TBE on the ISS.
DESIS will be able to detect changes in the land surface, oceans and
atmosphere; it will contribute to the development of effective measures
to protect the environment and climate. 2)3)

On July 20, 2015, TBE had entered in
an agreement with NASA for the provision of hyperspectral
remote-sensing imagery from an instrument to be based on the
International Space Station (ISS). 4)5)

Under the agreement, DLR will build
the DESIS (DLR Earth Sensing Imaging Spectrometer), a hyperspectral
instrument which Teledyne will integrate onto its ISS imaging platform,
the MUSES (Multi-User System for Earth Sensing). Teledyne is planning
to operate the instrument and retrieve remote sensing data which the
company will use for commercial applications and DLR will apply the
data to scientific research in atmospheric physics and Earth sciences.

MUSES itself is not a sensing
instrument; it is a platform for pointing a range of instruments that
could include spectrometers, multi and hyperspectral imagers, high
resolution panchromatic imagers, lidar, radar, magnetic sensors and
more. The platform can host four instruments simultaneously and offers
the ability to change, upgrade, and robotically service each
individually. One of the first instruments will be DESIS, a VNIR
(Visible/Near-Infrared) imaging spectrometer built by the DLR (German
Aerospace Center) to gather information on atmospheric physics and
Earth sciences. Ironically, the instrument will itself become an object
of study—DLR scientists will investigate the influence of the
space environment on remote sensing instruments once DESIS is returned
to Earth at the end of its mission. The fact that it can be returned to
Earth after its mission emphasizes once again some of the ISS’s
unique advantages. 7)8)9)10)

MUSES will be the first commercial
Earth-sensing platform on the ISS—commercial in the sense that it
will be designed, built, operated and managed by a commercial entity
TBE (Teledyne Brown Engineering), and research institutions and private
sector companies outside NASA will have the opportunity to mount their
instruments on the platform, specify where they would like to look and
own rights to the data gathered.

The MUSES platform provides accommodations for two large and two small hosted payloads (Figure 2).
MUSES is attached at the EXPRESS Logistics Carriers (ELC-4) starboard
of the ISS. It is a space-based, Earth-pointing platform providing
position sensing, data downlink, and other core services for each
payload attitude control. 11)

Figure 2:
MUSES platform with the four slots for different instruments. The
hyperspectral sensor DESIS will be integrated in one of the large slots
(TBE)

DESIS is one of the hosted
instruments on the MUSES platform, it has a mass of ~88 kg and is
integrated in one of the large containers.

MUSES is equipped with two gimbals,
thus allowing rotations around two axes up to ±25°
forward-backward, 45° backboard view and a 5° starboard view.
The pointing accuracy is better than 30 arcseconds, which corresponds
to about 60 m on ground at 400 km altitude. Together with the POI
(Pointing Unit) of the DESIS instrument, a ±40º along track
viewing is possible. The MUSES platform is also equipped with a star
tracker and a miniature inertial measurement unit providing attitude
measurement.

The MUSES platform is equipped with
a star tracker (sampling rate 10 Hz) and a miniature IMU (Inertial
Measurement Unit ) with a sampling rate 50 Hz providing a 10 Hz
attitude measurement after filtering. ISS GPS data provide position and
velocity vectors and time tags (sampling rate 1 Hz) serving as a master
time for the MUSES instruments with an accuracy of ±250
µs. The predicted viewing capability of MUSES, when operating at
the ISS orbit inclination of 51.6°, will enable the DESIS
instrument to scan about 90% of the populated Earth with a 3-5 day
average cadence. The daily download capacity is 225 GB.

MUSES will be
the first commercial instrument platform on the ISS. The MUSES platform
has a size of 85 cm x 85 cm and can accommodate four instruments. It
will be attached to a pivot arm on the side of the ISS facing Earth
during an astronaut spacewalk.

Figure 3: DESIS is the first instrument to be hosted aboard the MUSES platform (image credit: TBE) 12)

MUSES Facility Operations:

TBE will operate the DESIS
hyperspectral sensor on the ISS and will cooperate with DLR in using
the data in various areas, for instance systematic and applied
research. Nominal MUSES commanding is accomplished from the TBE TSC
(Telescience Support Center) in Huntsville, Alabama. A nominal weekly
file upload window is planned to allow regular scheduled updates to be
in sync with POIF (Payload Operations and Integration Function) normal
upload cycles. Any commands considered “critical” are
issued from the POIF, which manages all commands classified as
“critical”.

• August 20, 2018: The Center
for the Advancement of Science in Space (CASIS) and Teledyne Brown
Engineering (TBE) today announced a sponsored program up to $4.5
million, offering researchers the ability to propose flight project
concepts for the International Space Station (ISS) focused on remote
sensing and Earth observation. Within this opportunity, up to $1
million will be available for researchers to support sensor
development. Prospective awardees will utilize the Multi-User System
for Earth Sensing (MUSES) platform, developed and managed by TBE. This
funding opportunity will run through December 7, 2018. 15)

- Through this partnership, CASIS
and NASA intend to facilitate in-orbit access to the U.S. National
Laboratory on the space station. CASIS is the nonprofit organization
responsible for managing and promoting research onboard the ISS
National Laboratory.

- TBE developed the MUSES platform,
to host Earth-viewing instruments such as high-resolution digital
cameras and hyperspectral imagers and provide precision pointing and
other accommodations. MUSES can simultaneously host up to four
instruments and offers the ability to change, upgrade, and robotically
service those instruments. It also provides a testbed for technology
demonstration and technology maturation by providing long-term access
to the space environment on the space station.

- The ISS provides researchers a
unique vantage point of the Earth, orbiting at approximately 400 km
above the planet’s surface. Additionally, at an orbital
inclination of 51.6º and an orbital track recycle time of three
days, it provides researchers the ability to evaluate up to 95% of the
Earth’s habitable population on a daily basis.

DESIS is a hyperspectral camera that
records image data using an array of up to 235 closely spaced channels,
covering the visible and near infrared portions of the spectrum (450 -
1000 nm) with a ground resolution of 30 m. This multifaceted
information allows scientists to detect changes in ecosystems and to
make statements on the condition of forests and agricultural land.
Among other things, its purpose is to secure and improve the global
cultivation of food. The data from the ISS instruments will be
available quickly in the event of a catastrophe and can help rescue
teams operating on the ground to org anise their deployment. DLR and
TBE seek to combine the data from other MUSES instruments to develop
advanced methods for remote sensing of the Earth. Cooperation in this
scientific and commercial use will also promote hyperspectral
technologies for future satellites. 19)20)

Installation
on the ISS will also mean that the instruments can be brought back to
Earth after a service life of between three and five years to analyze
the influence of the space environment on the remote sensing
instruments. The platform with the DLR DESIS instrument is scheduled to
be mounted on the ISS by mid of 2017 and will, after a four month
commissioning phase, enter its operational phase at least until 2020.

DESIS was developed by DLR in a partnership with La Trobe University in Melbourne, Australia. 21)22)23)

DESIS is a hyperspectral instrument
is a pushbroom imaging spectrometer in a spectral range of 400 nm up to
1000 nm (VNIR) and based on a modified Offner design for the
spectrometer. The telescope is based on a TMA design. Beyond the
scientific mission goals, Teledyne will use the instrument for
commercial applications. The main difference between DESIS and most of
the hyperspectral design is that DESIS is equipped with a steering
mirror for BRDF (Bidirectional Reflectance and Distribution Function)
measurements; the minimum spectral resolution will be 2.55 nm which is
realized over 235 channels in an spatial resolution of 30 m GSD (Ground
Sample Distance). A 2D back illuminated CMOS (Complementary Metal Oxide
Semiconductor) detector array from BAE (CIS2001) is employed.

On-board programmable binning up to
a factor of four is possible. The SNR (Signal-to-Noise Ratio) for the
corresponding spectral sampling distances is shown in Figure 7.
The GSD (Ground Sampling Distance) depends on the flight altitude and
is about 30 m at nadir. This results in a swath width of about 30 km.
The electronic shutter mechanism is based on a rolling shutter: each
channel collects light during the same period of time, but the time
light collection starts and ends is slightly different for each
channel. As a result, each spectral channel integrates light over
slightly different surface areas on ground.

DESIS is equipped with a POI
(Pointing unit) consisting of two fixed and one rotating mirror in
front of the entrance slit that allows, by rotating one mirror, a
forward and backward viewing change up to ±15° w.r.t. the
nominal (e.g. nadir) view. The POI can be operated in a static mode
with 3º angle steps for the viewing direction and in a dynamic
mode with up to 1.5º change in viewing direction/s. This change in
viewing direction allows – besides normal Earth data takes
– acquiring experimental data to produce e.g. stereo or BRDF
(Bidirectional Reflectance Distribution Function) products and
continuous observations of the same targets on ground (FMC forward
motion compensation). The POI can also be operated in calibration mode
to minimize the external light fields and allowing on-board calibration
measurements.

Besides a pre-flight spectral,
radiometric and geometric calibration and characterization of the
instrument in laboratory, the instrument contains an on-board
calibration unit comprising different monochromatic and white light
LEDs between 400 and 1000 nm. The calibration unit is located close to
the POI mirror in front of the DESIS instrument, and allows the
illumination of the full spectrometer FOV with color and white light.
The calibration unit is characterized in laboratory and temperature
controlled in orbit within 1 K.

With an instrument mass of 35kg
(without container) and a size of ca. 400 x 400 x 500 mm, the
instrument is suitable for small satellites. The operation onboard of
the ISS shows the high performance which is possible with the
instrument design.

Figure 12: Cross section view of
the DESIS optomechanical system. The TMA is on the left, the Offner on
the right side (image credit: DLR)

The basic idea of MUSES is that all
parts of the instrument can be controlled and operated via TCP/IP in
the normal ISS network environment. This includes an on-instrument mass
memory unit which is able to store up to 512 Gbit of data without data
compression and an internal instrument control which is able to fulfill
all the imaging and calibration modes.

DLR’s motivation to do the
DESIS design together with Teledyne was to have the possibility to
generate a new class of hyperspectral data. Of course, the design is
able to generate standard data sets like most of the hyperspectral
designs. But in addition the design is able to penetrate the atmosphere
in different angles which allows a separation from volume to surface
BRDF effects. This will be used to control the atmospheric correction
to have a reproducibility of data results independent of the ISS
inclination.

DLR will use
the same technology to observe lightning by night not with the
necessary special resolution but with the high spectral resolution. In
this case DESIS will be operated in a forward motion compensation mode
based on an analog steering mirror control. DESIS can also be operated
in a forward motion compensation for other applications where the SNR
shall be enhanced.

DLR is also interested to
investigate work on florescence effects measurements in a surrounding
of mega cities as well in the country side.

Figure 14: Artist's view of the DESIS instrument on the MUSES platform of the ISS (image credit: NASA)

Development status:

• April 19, 2018: DLR (German Aerospace Center) and TBE (Teledyne Brown Engineering) are announcing the completion of the development and manufacturing process of the DESIS hardware.
Operating the DESIS (DLR Earth Sensing Imaging Spectrometer) on the ISS
(International Space Station) makes DLR the first user of the
revolutionary multiplatform system MUSES (Multi User System for Earth
Sensing) that was installed on board the ISS in 2017. The launch of the
DESIS joint venture is scheduled for summer 2018 from Cape Canaveral
and will be lifted into space by a SpaceX Falcon 9 rocket. 29)

- Hundreds of spectral channels for environmental monitoring:
DESIS will be DLR's first instrument for the analysis of hyperspectral
data on the ISS. "Hyperspectral sensing of Earth's surface is crucial
for environmental monitoring," summarizes Pascale Ehrenfreund, Chair of
the DLR Executive Board. "The continuous coverage of the VNIR (Visible
Near Infrared) spectral range makes DESIS a multi-purpose instrument,
which will help to gain new knowledge about agriculture, biodiversity,
geology and mineralogy, coastal zones, water ecosystems,
desertification and to detect changes in general."

- DESIS is a hyperspectral sensor
system with the capability of recording image data using 235 closely
arranged channels ranging from the visual to the infrared spectrum
(between 400 and 1000 nm) with a spatial resolution of 30 m while in
ISS orbit, at an altitude of 400 km. This data enables researchers to
detect changes in the ecosystem of Earth's surface to assess the status
of forests or agricultural areas, and therefore make yield predictions.
Thus, one of the tasks of DESIS is to secure and improve global food
cultivation.

- The spectral bands recorded by
DESIS are also ideal for determining the quality of water, in
particular of oceans and lakes. With the data, researchers are not only
able to determine water composition and pollutants, but can also
identify the causes of the contamination. Oil spills can be measured in
their extent and also in their thickness. The water content of soil can
also be analyzed using DESIS data. DESIS will be installed on the
multi-platform MUSES, which can host up to four Earth observation
instruments at the same time using the Canadian robotic arm (Canadarm2)
on the ISS. The platform was designed, developed and built by Teledyne
Brown Engineering. The instruments are mechanically locked in place and
have a separate power supply. This configuration makes the
International Space Station a universal instrument platform for Earth
observation that also enables replacement, repair and maintenance. Due
to the nature of this mission, it will also be possible to bring the
instrument back to Earth after its operational life of five to eight
years, in order to examine the effects of exposure to space conditions.

- High efficiency gain through cooperation with Teledyne Brown and utilization of the ISS:
"The mere fact that we did not have to build an entire satellite around
the DESIS instrument makes it a very cost-effective project," says Uwe
Knodt, program manager of DESIS. While DLR is responsible for the
construction of the instrument and the subsequent image data
processing, Teledyne Brown and NASA are responsible for transport to
the ISS and ensuring operations. Both organizations can greatly benefit
from one another. While TBD has the commercial licence to use the image
data, DLR provides expert imaging processing algorithms through licence
payments and will retain the legal right to use the data for scientific
applications. The use of MUSES and the ISS as an imaging platform allow
instruments to be ready and operational in a very short period of time
providing vital data to assist in scientific, humanitarian and
commercial missions.

- "Teledyne Brown Engineering is
thrilled with the progress achieved through the partnership with DLR,"
stated Jan Hess, President of TBE. "Our MUSES platform coupled with the
DESIS and other instruments will assist in the advancement of Earth
imaging, mapping, disaster recovery and agricultural assessments. Our
goal is to quickly populate each slot aboard MUSES to allow for maximum
coverage and data collection." This cooperation has had a fundamental
impact on the development of DESIS. The entire planning and
manufacturing process took just three and a half years – an
almost record-breaking achievement considering such a modern instrument
designed for low-Earth orbit.

- Together, DESIS and MUSES can
look forward, backward and sideways from the ISS. This high degree of
agility makes it possible to promptly provide information for relief
organizations in the event of a disaster. Collaboration in this type of
scientific and commercial operation is critical to the future prospects
of hyperspectral remote sensing technologies for satellite missions.
DLR and TBE want to leverage the data of DESIS and future MUSES
instruments to further improve Earth observation and to expand the use
of hyperspectral sensing in commercial applications.

Launch: The DESIS instrument
was launched on 29 June 2018 on the SpaceX CRS-15 (Commercial Resupply
Service-15) ISS logistics flight of the SpaceX Dragon capsule for NASA.
The launch vehicle was the Falcon-9 and the launch site was Cape
Canaveral, FL. 30)31)

Orbit: Near circular orbit, altitude of ~400 km, inclination = 51.6º.

Status of the DESIS instrument and sample observations

• June 2019: Teledyne Brown
Engineering (TBE) and DLR operate the imaging spectrometer DESIS that
is integrated in the Multi-User-System for Earth Sensing (MUSES)
platform installed on the International Space Station (ISS). 32)

- DESIS quantifies solar irradiance
reflected from the Earth surface as a response to their specific
condition. DESIS measurements greatly advance our ability to
characterize vegetation health and stress, water quality and pollution
as well as the Earth mineral resources. Thus, it supports the
management of agricultural and forest ecosystems, it helps to monitor
the biodiversity of our planet and it greatly enhances our
understanding of important carbon and water cycling processes.

- The DESIS instrument is realized
as a pushbroom imaging spectrometer spectrally sensitive over the
visible and near-infrared wavelength range from 400 to 1000 nm. The
optical design is based on the Offner-type grating spectrometer widely
used in imaging spectrometer designs. The Ground Sampling Distance
(GSD) at nadir view depends on the flight altitude of the ISS and is
about 30 m resulting in a swath width of about 30 km.

- DESIS Data can be tasked and
ordered for free for scientific purposes due to a general image data
announcement that will be open until the DESIS mission ends. It
requires a short description of the envisaged use of the data in form
of a proposal and a subsequent review process.

- The main objective of the
“DESIS Data Announcement of Opportunity” is to evaluate the
scientific capabilities of the DESIS data. It includes basic and
application-oriented research, including development and demonstration
of future applications to increase knowledge in science and does not
include commercial use. Scientific use of the DESIS data is not
restricted to any specific topic but it can include:

a) Basic and application oriented research,

b) Preparation and execution of government-funded education, research and development programs,

c) Development and demonstration of future applications for scientific and/or operational use,

d) Support of the MUSES mission (e.g. calibration, monitoring and validation) and

This general “DESIS Data Announcement of Opportunity” will be open until the DESIS mission ends.

• October 02, 2018: DLR (German
Aerospace Center) and Teledyne Brown Engineering presented the first
images of the DESIS hyperspectral Earth observation instrument at the
IAC (International Astronautical Congress) in Bremen, Germany. The
instrument was mounted to the exterior of the International Space
Station on 27 August 2018. 33)

- DESIS (DLR Earth Sensing Imaging
Spectrometer) has the highest performance of any hyperspectral Earth
observation instrument in space. From its 400 km-high orbit on the ISS
(International Space Station), this instrument acquires imagery on 235
closely spaced spectral channels with a ground resolution of 30 m. This
data is making it possible for scientists to gain very precise details
of changes to Earth's ecosystems. For example, they can use the
information obtained to assess the health condition of forests or
agricultural areas, and take such assessments as the basis for yield
forecasts. They can also use this data to monitor environmental
problems, thus providing the basis for devising measures to protect the
environment and resources.

- Hansjörg Dittus, DLR
Executive Board Member for Space Research and Technology, is
enthusiastic: "I am happy to say that the installation of DESIS –
and its commissioning – have proceeded smoothly. We have already
received and analyzed the first data – one month earlier than
anticipated. The wealth of data provided by DESIS is a fresh source of
knowledge for agriculture, biodiversity, geology, water-based
ecosystems and desertification. It is thus a very important instrument
for the environmental monitoring of Earth." The hyperspectral
instrument is a joint project between DLR and the United States company
TBE (Teledyne Brown Engineering), which owns the MUSES (Multiple User
System for Earth Sensing) Earth observation platform on the ISS. In the
course of this joint venture, DESIS will be providing DLR with data for
scientific purposes, while TBE will be responsible for the commercial
distribution of the hyperspectral data.

- David Krutz, project manager for
the DESIS instrument – and therefore the person responsible for
the construction of the instrument at DLR's Institute of Optical Sensor
Systems in Berlin-Adlershof – was equally enthusiastic: "DESIS
fulfils all expectations." For the initial commissioning, Krutz
travelled to TBE's facility in Huntsville, where the MUSES control
center is based. This gave Krutz the opportunity to work with TBE's
engineers to commission all of the subsystems immediately after
installation. Krutz summed it up as follows: "DESIS has successfully
passed all of its tests, and we were able to acquire the first image
amazingly fast. Seeing the first results with our very own eyes was
truly special."

- Rupert
Müller, project manager for the ground-level segment within DLR's
EOC (Earth Observation Center), had confidence in the system: "DESIS
has crucially expanded our know-how in the field of Earth observation.
Although there is still a lot to be done before DESIS is operational,
the first images are very promising. We can derive a fascinating extent
of information from the data." The team of scientists and engineers has
already been able to complete the first topic-centered analyses: it has
been possible to arrive at an assessment of the concentration of
chlorophyll a in the waters around the Falkland Islands from the
corresponding image. And researchers working near Huntsville have
succeeded in quantifying the content of colored dissolved organic
matter in the Tennessee River in Alabama, USA. These substances consist
of organic material, such as leaves and roots. Long organic molecule
chains are produced in the process of exposure to weathering, and these
chains can be well detected by DESIS on a range of spectral channels.
By merging its data with other Earth observation data, especially in
conjunction with some of the sensors that will be used on the MUSES
platform in the future, it will be possible to further improve the
quality of the data obtained by DESIS.

• August 27, 2018: After its
launch to the ISS (International Space Station) on 29 June2018, the
DESIS (DLR Earth Sensing Imaging Spectrometer) instrument was unpacked
and is now being prepared for installation on the Space Station's
exterior. 34)

- Its view of Earth will be
something special: The DESIS hyperspectral instrument has 235 spectral
channels to look at our planet and observe the changes in land and
water surfaces. On 27 August 2018 at about 21:00 CEST, the instrument
developed by DLR (German Aerospace Center) and equipped with a robotic
arm was taken out of the airlock of the ISS and installed on the MUSES
platform located on the space station's exterior. The hyperspectral
data obtained from space is expected to deliver information for
environmental monitoring, among other uses. "With this data, for
example, we can recognize whether the plants in a field on Earth are in
a state of stress at a given point in time from a distance of 400 km,"
says DLR project manager Uwe Knodt. "This will allow us to look at the
world from a new perspective and thereby benefit our society through
this view from space."

- Several conditions can trigger
stress on a plant: insufficient nutrients, unfavorable environmental
conditions and an inadequate water supply. The evaluation of DESIS
hyperspectral data through sophisticated algorithms will enable
conclusions to be drawn in this regard. Farmers, in turn, could benefit
from this data from space by learning at an early stage whether their
fields must be fertilized in a specific way, and which fertilizer their
plants need. The images acquired by the DLR instrument can also provide
information on the plants' stage of life, as well as the moisture
content of soil and plants. "As a result, the global cultivation of
food – and therefore the provision of food – can be
optimized," Knot explains. In addition, the instrument can deliver
information on the health of forested areas, identify the mineralogical
composition of specific regions, and record the constituent elements
and quality of oceans and lakes.

- The speed
of the images and the flexible angle for viewing Earth from space
enable other applications besides resource monitoring and environmental
surveillance. These potential uses include humanitarian aid, whereby
emergency rescue teams could receive valuable information from space
quickly and in a timely manner in the event of a disaster.

- DESIS is a joint project between
DLR (German Aerospace Center) and the U.S. industrial company Teledyne
Brown Engineering, which owns the Earth-observation platform known as
MUSES (Multiple User System for Earth Sensing). Within the framework of
this collaborative arrangement, DESIS is expected to provide DLR with
data for scientific purposes, while Teledyne Brown will take
responsibility for the commercial part of the hyperspectral data. Two
DLR institutes are involved in the project: The Institute of Optical
Sensor Systems built the instrument as part of the space segment
subproject, while DLR/EOC (Earth Observation Center) in
Oberpfaffenhofen is managing the ground segment subproject, which is
responsible for the reception of data, its processing and transfer into
applications.

• July 2, 2018: Teledyne
Technologies Incorporated announced today the successful launch of the
DLR Earth Sensing Imaging Spectrometer (DESIS) to the International
Space Station (ISS) on a SpaceX Falcon 9 rocket. Over the next three
months, the DESIS instrument, designed and built by the German
Aerospace Center (DLR), will be installed and tested on Teledyne Brown
Engineering’s Multi-User System for Earth Sensing (MUSES) aboard
ISS. 35)

• June 29, 2018: After arriving
at the ISS, the instrument will be unpacked and prepared for use on the
Multiple User System for Earth Sensing (MUSES) platform developed by
TBE. This platform can accommodate up to four Earth observation
instruments. DESIS will be moved into space through the airlock of the
Japanese Experiment Module (JEM). It will then be installed on the
MUSES platform using the Canadian robot arm Canadarm2. Installation
will take one day, and will be followed by commissioning, which
includes a system check and a pointing calibration test for sites on
Earth (Ref. 30).

The DESIS products are derived from tiled data takes of a size 1024 x 1024 pixels (i.e. about 30 x 30 km2),
which are generated within an automatic processing chain. Two identical
processing chains are implemented within DLR and the TBE Ground
Segment. The processing levels are following the definitions of ESA
(European Space Agency).

1) Product Level 0 (L0) – internal product:
Raw data after restoration of the chronological data sequence for the
instrument(s) operating in observation mode, at full space/time
resolution with all supplementary information to be used in subsequent
processing (e.g. orbital data, health, time conversion, etc.) appended.
Level 0 data are time-tagged. The precision and accuracy of the
time-tag shall be such that the measurement data may be localized to
accuracy compatible with the user’s requirements.

The Level 1A Processor collects
information from the different data streams, extracts and interprets
the information, and evaluates and derives additional information for
long term storage. The functionality of the Level 1A Processor
comprises the following tasks:

• Screening of the data
includes the inspection (comparison to reference tables) of all status
information, temperatures, currents and voltages, which are available
in the VC (Virtual Channel) of the DESIS instrument data.

• Extraction and evaluation of
DC (Dark Current) measurements before and after each datatake with 128
frames each. Mean and standard deviations of the DC frames are compared
to reference values.

• Preparation of Earth data
takes, which includes tiling of the data take (1024x1024 spatial
pixels), annotating metadata for further processing and quicklook
generation.

• Processing of calibration
data takes and publishing to the off-line calibration process, which
derives new calibration tables.

Systematic and Radiometric Conversion Processor (L1B):

The L1B processor corrects the data
for systematic effects and converts them to physical at-sensor radiance
or TOA values based on the calibration tables valid for the specific
time period. The correction includes the following tasks:

• Dead pixel flagging

• Dark signal correction using
linear interpolated dark signal values derived from the dark signal
measurements before and after each datatake

Orthorectification is the process to
generate map conform products by removing geometric distortions caused
by the sensor internal geometry, the satellite motion during data
acquisition and the terrain related influences. The DESIS L1C processor
produces orthoimages employing the technique of the rigorous model of
DG (Direct Georeferencing) . The LOS (Line-of-Sight) model forms the
basis of DG and is derived from the collinearity equation. LOS in this
context means the view direction of each pixel at any instance of time.

The sensor internal geometry of the
hyperspectral image will be extensively characterized in the laboratory
by highly accurate measurements of the direction angles of single
illuminated pixels (gravity center of the pixel) to the (adjusted)
collimator axis. For each pixel the two angles on object side
completely describe the internal camera geometry. This also includes
(possible) geometric keystone effects. The mounting angles with respect
to the attitude measurement system (body coordinate frame) are refined
in the commissioning phase by geometric calibration procedures due to
the gravity release and temperature influence. The star tracker
measurements are combined by Kalman filtering with the angular
measurements of the inertial measurement unit. These (unit) quaternions
are finally transformed from the ECI (Earth Centered Inertial) frame to
the ECR (Earth Centered Rotated) frame). A GPS provides the satellite
position (and velocity) with a rate of 1 Hz. The attitude and position
measurements are interpolated (e.g. Lagrange interpolation) for each
scan line of the image

Within an iterative process the
intersection points of the LOS vectors for each pixel and the DEM
(Digital Elevation Model) is determined and leads to 3D points in
object space.

The object points of the grid are
expressed in an Earth bound Cartesian coordinate frame and are
transformed to a user selectable map projection system [e.g. UTM
(Universal Transverse Mercator) with the zone derived from the center
coordinates of scene as well as the neighboring zones, geographic
projection]. Within the map projection system image resampling is
performed towards 30 m pixel spacing in case of UTM and 1 arcsec in
case of Geographic projection. Different selectable resampling methods
(e.g. nearest neighbor, bi-linear, cubic convolution) to generate the
final orthorectified products are offered to the customer.

The
geometric accuracy of the orthorectification is crucial for overlaying
the data with existing data sets, maps, or in geographic information
systems (GIS) and using them for evaluations like change detection, map
updating, and others like enhanced atmospheric correction using terrain
information (see chapter of Atmospheric Correction Processor).
Therefore, an improvement of the LOS model shall be achieved by GCPs
(Ground Control Points), which are extracted automatically from
reference images of superior geometric quality using image matching
techniques. As a global reference image database, the Landsat-8
panchromatic images with an absolute geometric accuracy of 12 m
circular error at 90% confidence level and 14 m ground resolution is
foreseen. 36)

In a first step, tie points between the uncorrected image and the reference image are determined by intensity based matching. 37)
Supplementing the height values, interpolated from the DEM, to the
found tie points a set of full qualified GCP are derived. In a second
step, the GCP sets serve as input to improve the LOS model parameters
within a least squares adjustment process. GCP outlier detection is
included in the matching processes itself as well as in the least
squares adjustment.

The feasibility of the approach to
extract GCP from reference images for a refinement of the DG model
within an operational environment has been successfully demonstrated at
different projects. For example, the production of two European
coverages by orthorectification of SPOT- 4 HRVIR, SPOT- 5 HRG and IRS
P-6 LISS III scenes. Figure 22 shows the
mosaic of the orthorectified scenes covering main parts of Europe. A
relative geometric accuracy of 10 m RMSE w.r.t. the reference images
have been achieved. About 450 GCP per 1000 km2 (about the size of one DESIS scene) have been extracted automatically.

The DESIS L2A processor performs
atmospheric correction of the images employing the well-known code of
ATCOR (Atmospheric/Topographic Correction for Airborne Imagery). 38)39)
Input for the atmospheric correction processor is the L1C product. For
the atmospheric correction over land a combined atmospheric and
topographic processing is possible, provided the geometric absolute
accuracy of the DESIS orthoimage is sufficient. A geometric accuracy
better than one pixel size is required for this combined topographic /
atmospheric correction in order to avoid artefacts caused by the
inaccurate co-registered DEM and orthorectified image.

The MODTRAN-5.3.3 (moderate
resolution atmospheric transmission) code is employed to model
properties of the solar reflective spectrum (from 400 to 2500 nm). It
supports a sufficiently high accuracy for the absorption simulation
(water vapor, ozone, oxygen, carbon dioxide etc.). It also includes a
rigorous treatment of the coupled scattering and absorption processes.
Moreover, it offers a set of representative aerosol models (rural or
continental, urban, maritime, desert). Therefore, MODTRAN-5.3.3 is
selected to compile a database of atmospheric correction LUTs (Look-up
Tables) with a high spectral resolution of 0.4 nm to enable the
processing of the 2.55 nm (binned 10.21 nm) channel bandwidths of
DESIS. This “monochromatic” or fine spectral resolution
database has to be resampled with the DESIS channel filter curves. The
advantage of compiling a “monochromatic” database is
twofold. First it gives the possibility of quickly resampling it with
updated spectral channel filter functions avoiding the necessity to run
time-consuming radiative transfer calculations for the solar and view
geometry pertaining to the acquired scenes and second to account for
spectral smile corrections.

The atmospheric correction accounts for flat and rugged terrain (Figure 23), and includes haze/cirrus detection algorithms.

Output products will be the ground
reflectance cube, maps of the aerosol optical thickness and atmospheric
water vapor, and masks of land, water, haze, cloud, shadow and snow. 40)41)

Inflight spectral, radiometric and geometric calibration:

Inflight calibration refers to all
measurements and data analyses aiming to assess radiometric,
spectrometric and geometric characteristics of the DESIS hyperspectral
instruments in orbit. DESIS will undergo extensive characterization and
calibration measurements before launch and will be re-calibrated after
launch by updating the calibration tables. 42)

The DESIS calibration unit (see Figure24 for the layout and Figure 6
for the location of the calibration unit) consists of a number of
different monochromatic and white LEDs with wavelengths between 400 and
1000 nm. The shape of the emitted light beam of the calibration unit
allows monochromatic and spectrally broad-band illumination within the
full spectrometer FOV. Pre-defined combinations of LED’s will be
operated at the same time to simulate different illumination scenarios.
The unit will be positioned in the spectrometer optical beam by the
corresponding position of the pointing unit mirror. The switched-off
calibration unit serves as dark reference.

Figure 24:
Principal design of the DESIS Calibration Unit, which shows three lines
with each eight monochromatic LED's and one white LED (image credit;
DLR-OS)

The DESIS instrument will be
operated by Teledyne and only Teledyne will receive the raw data from
the ISS. TBE has the exclusive right to license or transfer image data
for commercial use. For scientific and humanitarian purposes, DLR has
the right to task DESIS or request archived data. In these cases, and
if no conflicts with the commercial activities appear, Teledyne will
hand over the data to DLR. Therefore, Teledyne provides DLR a license
to the instrument data DLR receives for scientific and humanitarian
use. The parties will attempt to schedule DLR’s tasking requests
so that tasking is generally balanced throughout the calendar year.

For scientific purposes only, DLR
can share DESIS scientific data with other scientific organizations.
Any commercial use of these instrument data is prohibited without
Teledyne’s prior written permission. All end users of the
instrument data provided to DLR for scientific use will be required to
enter into a data license agreement among DLR and Teledyne. Scientific
use includes:

• basic and application oriented research

• projects by national and international educational or research institutions or by governmental institutions

• development and demonstration of future applications for scientific and/or operational use and

• preparation and execution of government-funded education, research and development programs.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).